1. Field of the Invention
The present invention relates to a mode switching method for a mobile station, which is suitable for mobile communication in which different service areas over-lap.
2. Prior Art
In conventional mobile communication in which there are over-lapping service areas, it becomes necessary to select a communication mode according to the service area. The selection of a communication mode is called mode switching. FIG. 1 is a conceptual drawing showing an example of a mobile communication system having over-lapping service areas. In FIG. 1, each communication mode of the mobile communication system A, B is mutually different, with a part of service area 1 for the mobile communication system A and a part of service area 2 for the mobile communication system B over-lapping. The mobile station MS in the service area 2 moves from position 31 to position 32 in the over-lapped area, and then moves to position 33 in the service area in which only mobile communication system A is applied. In conventional mobile communication, the mobile station MS autonomously recognizes the service area, in which it exists, on the basis of the received field strength. In this case, the mobile station MS observes signals which are transmitted from each base station of systems A and B, and then decides whether it is inside or outside a (particular) service area on the basis of the received field strength. In other words, when the received field strength PA at the mobile station MS of a signal sent from a base station in system A exceeds a fixed value pal preset as a system parameter, then the mobile station MS judges its position to be inside system A. Further, when received field strength PA is less than a fixed value pa2 preset as a system parameter, then the mobile station MS judges its position to be outside system A. Similarly, when the received field strength PB at the mobile station MS of a signal sent from a base station in system B exceeds a fixed value pb1 preset as a system parameter, then the mobile station MS judges its position to be inside system B. Further, when received field strength PB is below a fixed value pb2 preset as a system parameter, then the mobile station MS judges its position to be outside system B.
The received field strengths PA and PB of the mobile station MS present at position 31 in system B are
PA&lt;pa2 and PA1 PB&gt;pb1. PA1 PA&gt;pa1 and PA1 PB&gt;pb1. PA1 PA&gt;pa1 and PA1 PB&lt;pb2. PA1 PA&gt;pa1 and PA1 PB&gt;pb1. PA1 PA&gt;pa1 and PA1 PB&lt;pb2. PA1 PA&lt;pa1 and PA1 PB&lt;pb2. PA1 PA&gt;pa1 and PA1 PB&lt;pb2. PA1 PA&gt;pa1 and PA1 PB&gt;pb1. PA1 PA&gt;pa1 and PA1 PB&gt;pb1. PA1 PA&gt;pa1 and PA1 PB&lt;pb2. PA1 PA&gt;pa1 and PA1 PB&gt;pb1. PA1 PA&gt;pa1 and PA1 PB&lt;pb2. PA1 PA&gt;pa1 and PA1 PB&gt;pb1.
In this case, the mobile station MS selects a system B mode, because the mobile station MS exists in the service area of system B and outside the service area of system A. Next, when the mobile station MS moves from position 31 to an over-lapping area between both service areas, and then reaches position 32, the received field strengths PA and PB become
In this case, the mobile station MS can exist in both areas of systems A and B. Normally, the mobile station MS gives priority to the first selected mode, rather than selecting a plurality of modes simultaneously. Thus, the mobile station MS selects a mode in system B. Furthermore, when the mobile station MS moves from position 32 to a service area in system A only, and then reaches position 33, the received field strengths PA and PB of the mobile station MS become
In this case, the mobile station MS selects a system A mode, because it is present in the service area of system A but outside the service area of system B.
FIG. 2 is a conceptual drawing showing an example of a mobile communication system which has a service area included within another service area. In FIG. 2, service area 1 of the mobile communication system A includes service area 2 of the another mobile communication system B. This example, which assumes that system B has priority over system A in mode selection will explain the case where the mobile station MS moves from position 31 to position 32, and then moves from position 32 to position 33 successively.
The received field strengths PA and PB of the mobile station MS at the position 31, become
Although the mobile station MS exists in both service areas of systems A and B, the mobile station MS selects the mode of system B because system B has priority. Next, when the mobile station MS moves to position 32, the received field strengths PA and PB at the mobile station MS become
In this case, the mobile station MS selects a system A mode because the mobile station MS exists outside the service area of system B. Furthermore, when the mobile station MS moves to position 33, the received field strengths PA and PB of the mobile station MS become
Thus, the mobile station MS exists outside both systems A and B.
Further, when the mobile station MS once more moves to position 32 from position 33, the received field strengths PA and PB at the mobile station MS become
Thus, the mobile station MS is present inside the service area of system A, and outside the service area of system B. Therefore, the mobile station MS selects the system A mode. Furthermore, when the mobile station MS moves to position 31, the received field strengths PA and PB of the mobile station MS become
Although the mobile station MS is present inside both service areas of the systems A and B, the mobile station MS selects the system B mode because system B has priority.
However, in the above-mentioned conventional system, if service areas are discontinuous, or if the mobile station MS is present at the edge of a service area, repeated switching of modes occurs, giving rise to a phenomenon known as flapping. If there are many mobile stations MS in the above-described state, then location registration for mode switching is carried out repeatedly for each mobile station MS. Thus, the traffic on the control channel increases to or exceeds the limit of the processing capacity of the facility, so that congestion occurs.
An explanation of the aforementioned problem will now be made using FIG. 3. In FIG. 3, as in FIG. 1, the service area 2 of the mobile communication system B is included in the service area 1 of the mobile communication system A. In addition, a distinct service area 21 of system B is included inside service area 1 of system A. System B is assumed to have priority over the system A in mode selection.
This example will describe the case where the mobile station MS in service area 2 moves from position 31 to position 32, which is outside service area 2 but inside service area 1, and then moves from position 32 to position 33 inside the service area 21.
The received field strengths PA and PB of the mobile station MS at the position 31 become
In this case, although the mobile station MS can select modes of both systems A and B, the mobile station MS selects the mode of the system B which has priority. At this time, if temporary shadowing occurs, that is, if the mobile station MS enters the shadow of a building or a mountain, interrupting the electronic waves from the basestation, then the received field strengths PA and PB of the mobile station MS at position 31 become
As a result, the mobile station MS determines that it is outside the mode B area, i.e., outside service area 2, and then carries out location registration following mode switching. Once the mobile station MS recovers from the temporary shadowing, the received field strengths PA and PB of the mobile station MS become
Thus, the mobile station MS at position 31 carries out the location registration for system B again after switching the mode, because system B has priority.
Next, when the mobile station MS moves to position 32, the received field strengths PA and PB of the mobile station MS become
The mobile station MS at position 32 selects the mode of system A because the mobile station MS exists outside service area 2 of system B.
Furthermore, when the mobile station MS moves from position 32 to position 33, the received field strengths PA and PB at the mobile station MS become
In this case, although the mobile station MS is present in the service areas of both systems A and B, it selects the mode of system B which has priority. At this time, it is expected that the distinct service area 21 is a very narrow area. Thus, when the mobile station MS reaches position 32 by moving just slight distance, the mobile station MS switches to the mode of the system A, and must register its location in system A through the control channel of that system. Furthermore, when the mobile station MS moves slight distance to again, reach position 33, the mobile station MS switches to the mode of the system B, and must register its location in system B through the control channel of that system.
However, in the above-mentioned conventional system, in the case where there are many mobile stations MS which would give rise to flapping, then the control channel becomes subject to congestion due to traffic for location registration.
Furthermore, a large load is imposed on the network management system. Flapping also occurs in the case of a mobile station MS which exists on a boundary of service area 2 in system B. In addition, the fixed values pal, pb1 (in-level), and pa2, pb2 (out-level) are stored unchangeably as system parameters in the mobile station MS, so that they cannot be easily changed.